This application claims the priority benefit of Taiwan application serial no. 89101566, filed Jan. 29, 2000.
1. Field of Invention
The present invention relates to a method of forming a wide-viewing angle (WVA) liquid crystal display (LCD). More particularly, the present invention relates to a method of forming a wide-viewing angle, multi-domain vertical alignment (MVA), thin film transistor (TFT), liquid crystal display.
2. Description of Related Art
Liquid crystal display (LCD) has many advantages over other conventional types of displays including high picture quality, small volume occupation, lightweight, low voltage driven and low power consumption. Hence, LCD is widely used in small portable televisions, mobile telephones, video recording units, notebook computers, desktop monitors, projector televisions and so on. LCD gradually replaces conventional cathode ray tube (CRT) as a mainstream display unit. The biggest drawbacks of LCD are, however, its narrow viewing angle and its relatively high price.
At present, a number of propositions for manufacturing wide-viewing angle LCD is in the developing stage. The most widely adopted technique is the so-called pixel cutting method, or automatic domain formation (ADF). By controlling molecular orientation of the liquid crystal, a single pixel is divided into several domains so that the director of liquid crystal molecules in different domains has different tilt directions. Hence, viewing angle of the LCD is increased.
FIGS. 1A and 1B are side views showing the operation of a conventional multi-domain vertical alignment LCD. This type of LCD is proposed by Fujitsu Co. Ltd. of Japan in 1998. FIG. 1A shows the state of liquid crystal molecules inside the LCD when no external electric field is present or the electric field presence is lower than a threshold value. The color filter (CF) included glass panel 100 and the thin film transistor included glass panel 102 is parallel to each other. Protrusion elements 104 and 104 are formed on the inner surface of both the glass panel 100 and the glass panel 102. Negative type liquid crystal molecules 108 are vertically aligned between the glass panels 100 and 102 constituting a liquid crystal layer 110. Those liquid crystal molecules 108 close to the protrusion elements 104 and 106 tilt in specific direction due to local effects and resulting in pre-tilts.
FIG. 1B shows the state of liquid crystal molecules inside the LCD when an electric field above a threshold value is present. Due to the strong electric field, orientation of the negative type liquid crystal molecules 108 is changed such that director of the molecules is aligned in a direction vertical to the electric field. Liquid crystal molecules 108 near the middle portion of the liquid crystal layer 110 are pre-tilted and the electric field fringing the protrusions 104 and 106 is non-uniform. Hence, within the same pixel, molecules on each side of a protrusion will tilt oppositely and have different molecular alignment. The protrusions 104 and 106 within a pixel divide the pixel into two or more domains. In other words, a multi-domain pixel is formed and viewing angle of LCD is improved.
FIG. 2A is a schematic top view showing one of the pixels of a second type of conventional multi-domain vertical alignment LCD. FIG. 2B is a cross-sectional view along line 2Bxe2x80x942B of FIG. 2A.
As shown in FIG. 2B, the structure includes two glass panels 200 and 202 running parallel to each other. A liquid crystal layer 204 is formed between the glass panels 200 and 202. The structure is very similar to the one in FIG. 1A in that the inner surface of the upper glass panel 200 has protrusions 206 thereon. A transparent electrode 208 is formed on the inner surface of the lower glass panel 202. The transparent electrode 208 further includes some slits 210 that serve as virtual protrusion. The protrusion 206 and the slit 210 are alternately positioned.
As shown in FIG. 2A, the single pixel structure has a data line 212 and a scan line 214 around the periphery of the transparent electrode 208. The data line 212 and the scan line 214 are connected to the source terminal 218a and the gate terminal 218c of a thin film transistor (TFT) 218 respectively. The drain terminal 218b of the thin film transistor 218 is connected to the transparent electrode 208. Control signals are transmitted to the source terminal 218a and gate terminal 218c of the thin film transistor 218 via the data line 212 and the scan line 214 respectively. Orientation of liquid crystal molecules inside each pixel is changed to display an image by employing an active matrices drive. The common line 216 that serves as an electrode for the storage capacitor Cs is located between the lower glass panel 202 and the transparent electrode 208. Moreover, the common line 216 passes out through the mid-portion of the transparent electrode 208. Through the alternately positioned protrusion 206 and slit 210 on the inner surface of different glass panels of a pixel, the pixel is divided into four different domains so that viewing angle of the LCD is increased.
The protrusions are formed by spin-coating a layer of photoresist material over the glass panel, and then performing photolithographic operation using a photomask. Hence, to form protrusion elements on both the upper and the lower glass panel, two photolithographic operations have to be conducted. However, uniformity and pitch distances between protrusion elements are difficult to control using the conventional method.
In addition, the upper and the lower glass panels must be meticulously aligned when they are assembled to form a LCD. Since both the upper and the lower glass panel have protrusion elements or one with protrusion elements and other with slits, any misalignment of the glass panels is likely to affect brightness of the LCD. Occasionally, the entire LCD module may have to be scrapped due to protrusion element misalignment. Hence, process window for aligning glass panels is tight. Furthermore, since the electric field around the protrusion elements and the slits of the glass panels are weaker than other transparent electrode regions, liquid crystal molecules above these regions may not re-orient themselves in the presence of a strong pixel voltage. Therefore, these regions become permanently dark. In other words, the protrusion element regions and the slit regions will occupy a portion of the light passing area within the pixel to form a dark region. Consequently, aperture ratio of a pixel is reduced leading to inferior pixel quality.
In general, the transparent glass panel of a thin film transistor LCD normally has non-transparent metal electrodes. These metal electrodes can be utilized as a self-aligned photomask in a backside exposure (BSE) method for producing a multi-domain vertical alignment LCD. FIGS. 3A through 3D are schematic cross-sectional views showing the steps for producing the glass panel of a multi-domain vertical alignment LCD using a conventional backside exposure method.
As shown in FIG. 3A, a glass panel 300 having a non-transparent metal electrode 302 thereon is provided. A photoresist layer 304 is formed over the front face of the glass panel 300 so that the metal electrode 302 is also covered. As shown in FIG. 3B, ultraviolet light is shone from the backside of the glass panel 300 onto the photoresist layer 304 using the metal electrode 302 as a photomask. Here, no other photomask is used. If the photoresist layer 304 is formed from a positive type of photoresist material, protruded sections 306 that covers the metal electrodes 302 as shown in FIG. 3C are formed after backside exposure, development and baking. On the other hand, if the photoresist layer 304 is formed from a negative type of photoresist material, trenches 308 that exposes the metal electrodes 302 as shown in FIG. 3D are formed after backside exposure, development and baking
This type of multi-domain vertical alignment LCD utilizes signaling bus lines directly as a photomask for backside exposure. Hence, a photomask production step is saved. However, because signaling bus lines are normally placed around the periphery of a pixel forming a grid layout, ultimately produced protruded sections are limited to the peripheral regions of the pixel. Hence, this method is unable to control the orientation of liquid crystal molecules within a pixel. When an image is shown on the LCD, dark hairline is likely formed inside pixel display area leading to a wholesale lowering of pixel transmittance and performance.
Accordingly, one object of the present invention is to provide a method of forming wide-viewing angle liquid crystal display (LCD) that utilizes backside exposure technique to form self-aligned protruded elements on a LCD glass panel without any other photomask. Furthermore, the method also utilizes the circuit layout of the LCD to make the protruded elements and the electrodes of storage capacitor overlap. Hence, light-passing region, brightness, aperture ratio and lighting efficiency of a pixel all increase, and response time and display characteristics of the LCD are improved. In addition, protruded elements and slits are formed on the same LCD glass panel. Therefore, processing window is increased and cost of production is lowered.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a wide-viewing angle liquid crystal display. The wide-viewing angle LCD includes a first glass panel and a second glass panel running parallel to each other and a liquid crystal layer between the two glass panels. A common line, a transparent insulation layer, a transparent electrode layer, first protrusion elements, a first alignment layer are sequentially laid over the inner surface of the first glass panel. A color filter, a transparent conductive film, second protrusion elements, a second alignment layer are sequentially laid over the inner surface of the second glass panel. Layout of the common line is used as a photomask in backside exposure for forming the first protrusion elements. Therefore, the first protrusion elements overlap with the common line in location and area. The first protrusion element and the second protrusion element are formed in alternate positions along the inner surface of the upper and the lower glass panel respectively.
The invention also provides a second type of wide-viewing angle LCD. The wide-viewing angle LCD includes a first glass panel and a second glass panel running parallel to each other and a liquid crystal layer between the two glass panels. A common line, a transparent insulation layer, a transparent electrode layer, protrusion elements, a first alignment layer are sequentially laid over the inner surface of the first glass panel. A color filter, a transparent conductive film, a second alignment layer are sequentially laid over the inner surface of the second glass panel. There are slits along the transparent insulation layer. A portion of the layout of the common line is used as a photomask in backside exposure for forming the first protrusion elements. Therefore, the protrusion elements overlap with the common line in location and area. The protrusion elements and the slits in the transparent insulation layer are formed in alternate positions along the inner surface of the upper glass panel and the transparent insulation layer respectively. In addition, signaling bus line and transparent protection layer can be inserted between the transparent insulation layer and the transparent electrode layer. A portion of the signaling bus line layout overlap with the common line so that the protrusion elements, a portion of the signaling bus line and a portion of the common line overlap in location and area.
The invention provides a method of forming a wide-viewing angle liquid crystal display. A first glass panel is provided. A common line having a pattern of desired first protrusion elements, a transparent insulation layer and a transparent electrode layer are sequentially formed over the front surface of the first glass panel. A photoresist layer is formed over the transparent electrode layer. Using the common line as a photomask, backside exposure of the photoresist layer and photoresist development are carried out to form the first protrusion elements. A second glass panel is next provided. A color filter film and a transparent conductive film are sequentially formed over the front surface of the second glass panel. Second protrusion elements are formed above the transparent conductive film such that the second protrusion elements are alternately positioned with respect to the first protrusion elements on the opposite side of the first glass panel. A first alignment layer and a second alignment layer are formed over the front surfaces of the first glass panel and second glass panel respectively. Hence, the protrusion elements on the inner surfaces of the respective first and second glass panel are covered. The first and the second glass panel are assembled. The first and the second glass panel are aligned such that the first and the second protrusions are alternately positioned on the front surface of the first and the second glass panel. Finally, liquid crystal material is injected into the space between the first and the second glass panel to form a liquid crystal layer.
The invention also provides a second method of forming a wide-viewing angle LCD. A first glass panel is provided. A common line having a pattern of desired protrusion elements, a transparent insulation layer having slits at alternate positions relative to the protrusion locations and a transparent electrode layer conformal to the transparent insulation layer are sequentially formed over the front surface of the first glass panel. A photoresist layer is formed over the transparent electrode layer. Using the common line as a photomask, backside exposure of the photoresist layer and photoresist development are carried out to form the protrusion elements. A second glass panel is next provided. A color filter film and a transparent conductive film are sequentially formed over the front surface of the second glass panel. A first alignment layer and a second alignment layer are formed over the front surfaces of the first glass panel and the first surface of the second glass panel respectively. Hence, the first protrusion elements of the transparent conductive film and the slits are covered. The first and the second glass panel are assembled. Finally, liquid crystal material is injected into the space between the first and the second glass panel to form a liquid crystal layer.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.